fix: OpenAPI - WebMessage response as oneOf schema [DHIS2-18251]

[jbee]

Summary

Goal was to fix the OpenAPI reflection of the WebMessage.response property.

This has always been a special handling situation around WebMessageResponse. The issue was that the classes were located in modules that could not be referenced and so special handling was hard-coded.
This PR removes this special handling and details the property with all the possible types (oneOf in OpenAPI terms).

This now looks like this

To get there a cleanup was required which mainly moved files around and removed special handling.

• introduced BasicWebMessage
• WebMessage itself is not a WebMessageResponse
• removed AbstractWebMessageResponse
• all implementations of WebMessageResponse are in web-api module
• WebMessageResponse always states its responseType based on the Class.getSimpleName() (existed as a concept before but wasn't used/implemented consistently)
• removed the "plain before" concept as it was only relevant for 2.37- which at this point is EOL and it was in the way of the cleanup
• Exceptions that become web messages now can link to BasicWebMessage (WebMessage has web layer refs and cannot be used and fields added would be null anyhow for errors)

Most classes were moved without changing the package to reduce the risk of breaking something odd.
A few exceptions were moved into another package when it felt safe to do so.
Long term we have to cleanup the packages in another PR.

Manual Testing

Open /api/openapi/openapi.html?domain=DataElement#WebMessage and compare to image above

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
5 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.8% Duplication on New Code

See analysis details on SonarCloud